The main circuit function performed by a Transient Voltage Suppressor (TVS) is to protect the active elements of the circuit from unexpected transient voltage spikes. These sudden and unexpected transient voltage spikes can be generated by natural causes (lightning being the most common one) or by other high power equipments, operating in close proximity to the sensitive electronic circuit.
The TVS function can be achieved with various devices, most common ones being Metal Oxide Varistors (MOVs), Zener or P-N Junctions operating in avalanche (Semiconductor Avalanche Diodes, or SADs) and by Crowbar Elements (like Gas Discharge).
TVS devices can be made to block (clamp) voltage in one direction (one polarity of the applied voltage) and in this case they are called “unidirectional TVSs” or they can block (clamp) voltages of both polarities (“bi-directional” TVS).
A TVS element, connected in parallel to the circuit to be protected and having a set voltage at which the voltage across the TVS does not increase any further, regardless of how much current flows through it, is the ideal device to ensure continuous operation of the electronic circuit. FIG. 4 illustrates in a very generic circuit the electrical function performed by a TVS to “clamp” the voltage spike induced accidentally by an outside voltage source by “clipping” it to the rated voltage of the TVS. A “transient current” starts to flow through the TVS, the magnitude of this current being determined by the difference between the peak voltage spike and the rating voltage of the TVS divided by the “avalanche series resistance” of the TVS. It becomes apparent to a skilled person that a low series resistance will keep the clamp voltage constant during the current decay, a feature of great importance, but at the same time the power and temperature stress applied to the TVS will be very high in the case of a low series resistance.
Material properties of SiC, like wide band-gap, high electric field, high saturated drift velocity and high thermal conductivity, strongly recommend SiC for power devices. However, a need exists for a suitable structure for implementation of a TVS device in SiC.